Blender not importing openings from doors or windows from IFC file
After painful hours of trial and error, I found the solution (or at least the problem, to some extent).
The problem comes from the way the file is exported to IFC format from Revit. Even though on the IFC viewer everything looked fine, there's something on Revit that does make a difference when exporting: the IFC version.
The version I had set was IFC 2x3 Coordination View 2.0, because I understood it's very reliable and because it exports objects, if not with textures, at least with material names assigned, which makes applying textures on Blender very easy.
However, there were issues with walls and opennings. In some cases, the opening was imported properly, in other cases it wasn't.
Now, a side note about this: The openings were actually imported, but they didn't go all the way through the wall. I tried playing with the width of wall and window and door families, but I never got a consistent result and was running out of time (an patience!)
The IFC version that imported the openings and all elements without a problem, was IFC4 Reference View, which I've heard is not as widely accepted as IFC 2, but it worked for me. Downside is that objects are imported without material names assigned, BUT, the material names list is imported.
I'm moving from there and assigning materials object by object, since it was more important for me to have walls with proper openings.
If somebody has a better solution, do say!